my %map = (
	F => "|",
	S => "^",
	T => "&",
	R => "~",
	E => "\\"
);

$_ = "\\E\\S\\";
s/\\([FSTRE])\\/$map{$1}/g;
print "s///g: [$_]\n";

$_ = "\\E\\S\\";
1 while s/\\([FSTRE])\\/$map{$1}/;
print "1 while s///: [$_]\n";

__END__
s///g: [\S\]
1 while s///: [^]